The base charges for colocation services are generally based upon one of two items. The first of these is the amount of space that a server will take at the colocation facility. The two most common terms used when referring to the amount of space that is used is rack or U's and cabinets.
Racks
In order to standardize computer and networking hardware located in machine room facilities, the industry standardized on a 19" rack design. This is essentially a form of cabinet or stand that has the horizontal mounting hardware spaced 19" apart. This allows for all hardware manufacturers to design their equipment to be easily installed inside of the racks.
The overall height of the items in the rack can vary, but a standard size was also chosen to determine the height of components. A standard height unit or U for a rack mount case is roughly 1.75 inches high. Components are then labeled with their height in terms of the number of U's. A standard industry server size is typically 1U while most networking gear such as switches will be 2U and drive arrays upwards of 4U or greater.
The number of U's required to mount the hardware into the colocation provider will determine the base price of the server. A single 1U server will typically be less expensive than a larger 2U system. Now to use full sized expansion cards found in most desktop computer systems, the server would need to be in a 4U server case that is significantly more expensive. This means that when determining the costs of the server, one must also look at the costs and equipment necessary to fit in that space.
Cabinets
A cabinet is typically the term used to refer to a full sized rack or about 47U in space for equipment mounting. A cabinet that has additional security such as a locking door panel might be referred to as a cage or closed cabinet and provides an additional level of security to the hardware installed in the facility. Cabinets are typically rented out by larger corporations that need a fair amount of space for multiple servers and storage arrays.
Partial cabinets are also available at most colocation facilities. Typically they are either a 1/2 cabinet (about 24U) or 1/4 cabinet (about 12U). This is still a large amount of space for hardware to be installed and will cost a fair amount of money. The reason for mentioning this is that most providers will give quotes for a 1/4 cabinet if the hardware is not rack mountable. For a single server, this can be a huge cost over migrating to a dedicated rack server case that is even as large as the 4U size.
Amps
In addition to the amount of space that the server may take up, colocation providers also need to provide power to the systems installed on their site. As a precaution for power outages, most providers have some form of uniterruptable power supply attached to all the system. In order to insure they have the power backup power, they need to know the power usage by the systems installed on site. This is where amps come into play.
Typically, a provider will allot a certain number of amps to correspond to the amount of space that is rented in the facility. In most cases it is between 1 to 2 amps per U that is rented. Most server hardware should easily fall within these power specifications but if the system does use more, expect to pay extra for the additional power allotment.
